The Marine Corps date all the way back to the 1770s when congress resurrected the two battalions of marines. Originally the marines were to supply guards to warships. Over the 20th century the corps transformed into a multi-function organization that combines ground and air combat units into a maritime force, trained to come from the seas to fight on the land.
Although you might not have known or even thought about it, the marines have been there in so many wars like WW1 , the Mexican-American War, Pearl Harbor, and they had a lot of involvement in Vietnam, etc.
The marine corps hymn is one of the most recognized songs today and is the oldest of the service songs of our country. The author of the hymn is unknown. The words are known to have been written by a marine in 1847, 20 years before the music to the hymn was made by Jacques Offenbach a french composer. When the hymn is played marines stand at attention.
There are multiple things you can focus on and do in the marines today. You can do things like artillery, infantry, expeditionary, recruiting, and many more. Those are just a phew of the many many things you can focus on.